Why not use a discount to which one is legitimately entitled?

If the ticket policies were based on age alone with a clear line whether 18, 12 or somewhere else, it would be easy to obey them, and if challenged about a child who looks old for her or his age, it could be backed up with ID proof of age. But the height requirement makes enforcement confusing.

By the letter of the rules, a girl of 17 who would stand 149 cm tall without her high heels and hairdo is entitled to a child ticket. But if challenged about it - age can be proven by ID, how easy is proving the true height going to be?